This invention relates to availability of active devices in a coaxial cable plant and in particular to increasing the availability of RF amplifiers in the coaxial cable plant without lowering network efficiency.
Signal transmission systems using cable transport media tend to be highly lossy compared with such transmission systems as over the air radio media. Cable is generally intended to be coaxial cable, which defines a specific wired technology. These systems normally use trunk cables to transmit from a headend source to distribution nodes. Distribution cabling connects the nodes to drop points where drop lines are connected to a receiving customer. It is becoming common to replace the trunk lines with optical fiber cable due to its wider bandwidth and lower loss. The distribution or feeder coaxial cabling however is largely already in place and remains the transmission media of choice. Due to the many tap off points of a distribution cable the use of many cascaded amplifiers is needed to maintain a desired signal strength and quality. Failure of any amplifier can seriously degrade the signal integrity, down stream from the failure, ranging from a poor signal-to-noise ratio (SNR) up to an outright failure of the signal transmission. Hence, continuous provision of quality amplification is very important.
Provision of needed amplification may largely be secured by parallel combined or switched redundancy whereby continuously operating amplifiers are arranged in parallel or in backup switched redundancy systems where operative amplifiers are substituted for failed amplifiers. Combined paralleled redundancy allows two amplifiers to operate continuously resulting in reduced requirements on the individual amplifiers. However, the coupling in a parallel mode requires the use of a combining device. Should one of the amplifiers fail the combining device couples to only one active path and the inactive path termination introduces added noise into the cable system. This failure mode not only reduces the amplification gain it also introduces a loss due to a parallel path termination into the system. All subsequent cascaded amplifiers amplify this noise.
Switched redundancy systems provide amplification of the same quality but require extensive insertion into the system of inactive amplifiers, which in a capital-intensive cable system is undesirable. It is desirable to utilize the benefit of switched redundancy without retrofitting the amplification system to remove the combined redundancy amplifier arrangements.
Advantages of the parallel combined redundancy and of the switched redundancy are provided by use of a broadband switch/combiner that flexibly allows active amplification devices to be either switched combined or parallel combined. Sensing circuitry monitoring the amplifier""s operating state control the switch/combiner so that in the event of an amplifier failure the maximum loss incurred is that of the lost amplifier only, as opposed to the losses incurred in present systems.
In a particular exemplary embodiment, parallel combined redundancy and switched redundancy are provided for active RF devices (i.e., amplifiers) in a cable plant system with a switch/coupler circuit of passive magnetic devices and switches that respond to an amplifier failure by switching the circuit into a combined or switched redundancy mode as needed to maintain a suitable dynamic range of operation. In either redundancy mode, the switch/coupler circuit operates with a constant 3dB reduction of dynamic range.